1. Field of the Invention
The present invention relates to decoupling technology for coils in a magnetic resonance imaging system and, particularly, to a coil for a magnetic resonance imaging system.
2. Description of the Prior Art
Magnetic resonance imaging (MRI) systems have important applications in fields such as clinic diagnosis etc. Dependent on the intensities of the main magnets in MRI systems, the MRI systems can be classified roughly into three major categories of low field (less than 0.3 T), middle field (0.3 T to 1.0 T) and high field (larger that 1.0 T). Correspondingly, radio frequency signal receiving coils can also be classified into low field receiving coils, middle field receiving coils and high field receiving coils. In addition, according to the shapes of receiving coils, the receiving coils can be classified into volume coils and surface coils. Generally speaking, a volume coil can accommodate a part of a human body to be scanned for imaging (for example, a head or a limb) into the coil; while a surface coil is for placing on the surface of a part of a human body to be scanned for imaging, such as spine, eye, abdomen, etc.
With the development of receiving coil technology, most of the receiving coils composed of a number of coil units use the phased-array technology, so the coils using the phased-array technology can be referred to as phased-array coils. Furthermore, with the development of receiving coil technology, the number of the coil units in the phased-array coils is continuously increasing, and the signal-to-noise ratio (SNR) and uniformity of the image obtained thereby are significantly improved. Negative effects associated therewith, however, are that the coupling level among various coil units is also increased, the modes of coupling are becoming more and more complicated, and the number of coil unit pairs requiring decoupling treatments is becoming larger and larger. If the coil units are inadequately decoupled, it will produce negative influences on the noise matching of the preamplifier relevant to the noises between the coil units, thus putting the signal-to-noise ratio of the image obtained from the coil under considerable adverse influence.
The conventional modes for decoupling coils can be generally classified into the following types.
The first mode is to have partial overlap between two adjacent coil units. In FIG. 1, the coil units 101 and 102, 102 and 103, 103 and 104, 104 and 105, 105 and 106, 106 and 101 are respectively adjacent to each other, and there is an overlapped part between every two adjacent coil units. Taking the adjacent coil units 101 and 102 as an example, due to the existence of the overlapped part, it makes the net magnetic field generated by the coil unit 101 in the coil unit 102 be zero, therefore the magnetic coupling is also zero, so as to achieve the decoupling, and vice versa. If this mode is used, each of the coil units can be independent from each other, and it is not necessary for two coil units to have electrical connection in a circuit. Currently, in MRI systems with high field (generally horizontal field), most receiving coils use this mode. This mode can produce good decoupling effects to adjacent coil units, however, since there is no way to make those non-adjacent coil units partially overlapped, this mode is not suitable for the decoupling between non-adjacent coil units.
The second mode is to decouple by a pre-amplifier decoupling method. In the surface coils, the coupling between non-adjacent coil units is generally weak, and it can be decoupled by way of the preamplifier decoupling method, and it is not necessary to employ a decoupling structure design in the coil. However, in the volume coils the coupling between non-adjacent coil units is generally stronger, it is far from enough by only relying on the preamplifier decoupling. It is thus necessary to introduce a dedicated decoupling configuration to eliminate the coupling between the non-adjacent coil units.
The third mode is to introduce a capacitance circuit or an inductance circuit into the electric circuit to achieve decoupling. According to the different polarities of inductive electric potential, a corresponding capacitance circuit or inductance circuit is introduced to counteract the generated inductive electric potential, so as to achieve the purpose of decoupling. Upon using this mode, it is necessary to establish electric circuit connections among various coil units to be decoupled, otherwise the decoupling capacitors or inductors cannot be introduced. Currently in the middle and low field (generally vertical field) MRI systems, since basically all coil units (for example, loop coils or saddle coils) have electrical connections and the number of coil units is relatively small, it is easy to introduce a capacitance or inductance circuit. Moreover, in the low and middle fields, the frequency of RF signals is not high, various RF effects such as loss etc. can be ignored, therefore, this mode is widely used in the MRI systems of middle and low fields.
However, for the high field (usually horizontal field) receiving coils, between the coil units other than the adjacent coil units there are usually no connection points suitable for connecting a capacitance or inductance circuit, if a capacitance or inductance circuit is introduced between two coil units spaced relatively far away (for example, between the coil unit 101 and the coil unit 104 shown in FIG. 1), it will certainly bring about some loss and thereby reduce the signal-to-noise ratio. In the case of the number of coil units being large, if the decoupling is carried out entirely by using capacitance circuits or inductance circuits, then it is necessary to add many additional decoupling circuits, and these decoupling circuits will produce considerable adverse influence on the matching of the corresponding coil units. Therefore, after the decoupling it is necessary to perform matching and this matching will certainly affect the original decoupling treatment. Further decoupling then is necessary after the matching. Therefore, in this case, it is necessary to carry out repeated decoupling and matching, and the finally obtained coil is certainly a complicated and unstable coil.
In summary, for the volume coils of high field (generally horizontal field) MRI systems that are composed of a number of coil units, the above first mode can be used to perform decoupling between adjacent coil units of this type of coil, but there is no solution with good decoupling effect for non-adjacent coil units.